4.1.1.6 BLE Multilink Transparent UART

Introduction

This section helps users to create a multilink central device and send/receive characters between connected BLE devices over Microchip proprietary Transparent UART Profile. The multilink central enables users to connect multiple peripheral devices to a central device. The central and peripheral devices in this tutorial are the WBZ351 devices.

Users can choose to run the precompiled application example .hex file on the WBZ351 Curiosity Board and experience the demo or go through the steps involved in developing this application from scratch.

It is recommend to follow the examples in order, by learning the basic concepts first and then progressing to the more advanced topics.

Recommended Reading

1. BLE Software Specification

2. BLE Connection(central)

3. BLE Transparent UART(central)

Hardware Requirement

SDK Setup

• Getting Started with Software Development

Software

• Tera Term

Smartphone App

None

Programming the Precompiled Hex File or Application Example

Programming the .hex File using MPLAB X IPE

1. Central Device – Precompiled .hex file is located in "<Harmony Content Path>\wireless_apps_pic32cxbz3_wbz35\apps\ble\building_blocks\central\profiles_services\multilink\hex" folder
2. Peripheral Device – Precompiled .hex file is located in "<Harmony Content Path>\wireless_apps_pic32cxbz3_wbz35\apps\ble\building_blocks\Peripheral\profiles_services\peripheral_trp_uart\hex" folder

3. Follow the steps mentioned here

Programming the Application using MPLAB X IDE

1. Follow steps mentioned in the Running a Precompiled Example section.
2. Central Device – Open and program the application example "central_ml_trp_uart.X" located in "<Harmony Content Path>\wireless_apps_pic32cxbz3_wbz35\apps\ble\building_blocks\central\profiles_services\multilink\firmware" using MPLAB X IDE
3. Peripheral Device – Open and program the application example "peripheral_trp_uart.X" located in "<Harmony Content Path>\wireless_apps_pic32cxbz3_wbz35\apps\ble\building_blocks\peripheral\profiles_services\peripheral_trp_uart\firmware" using MPLAB X IDE

For more details on finding the Harmony content path, refer to Installing the MCC Plugin

Demo Description

Upon programming the demo application, central device (WBZ351) will start scanning for near by peripheral devices to connect. After a connection has been made data can be sent back and forth over UART between the two devices that are connected. The central device continues scanning until 100 secs and allows new peripheral devices that are advertising to join. For this example we are going to demonstrate three BLE links

                    ------------------Central Device------------------
                    |                      |                         |
                    |                      |                         |
            Peripheral Device       Peripheral Device        Peripheral Device

Demo application prints the status messages "Scanning" at the initiation of the scan process, "Connected!" upon successful connection, and "Disconnected" when the connection is lost, within a terminal emulator interface.

Application data to be sent to the connected peripheral device must be entered in the terminal emulator.

Testing

For more details on changing the device address, refer to the “Set PUBLIC Device Address” in BLE Transparent UART section.

Demo Experience when Using Four WBZ351 Curiosity Boards, Three Configured as Peripheral and One Configured as Central Device

After connection establishment, both the Peripheral device (Board2/3/4) and Central device (Board1) will display "Connected!" message on respective terminal windows.

Users can now start sending data back and forth between the central and peripheral device using the terminal emulator. Characters entered on any peripheral device's (Board2,3,4) terminal emulator will appear on central device's (Board1) terminal emulator. Characters entered on central devices terminal emulator will appear on peripheral devices emulator in a round-robin fashion without priority. For example in order to send character "a" to all peripheral devices, "aaa" needs to be entered on terminal emulator of the central device

Developing this Application from Scratch using MCC

This section explains the steps required by a user to develop this application example from scratch using MCC

1. Create a new MCC Harmony Project. For more details, refer to Creating a New MCC Harmony Project.

2. Import component configuration: This step helps users setup the basic components and configuration required to develop this application. The imported file is of format .mc3 and is located in the path "<Harmony Content Path>\wireless_apps_pic32cxbz3_wbz35\apps\ble\building_blocks\central\profiles_services\multilink\firmware\central_ml_trp_uart.X".

   For more details on importing the component configuration , refer to Importing Existing App Example Configuration

   Note: Import and export functionality of Harmony component configuration will help users to start from a known working setup of MCC configuration

3. Accept Dependencies or Satisfiers, select Yes

4. Verify if the project graph window has all the expected configuration. as illustrated in the following figure:

   

Verifying Scan,Connection and Transparent UART Profile Configuration

1. Select BLE Stack component in project graph, to open component configuration and configure as illustrated in the following figure.

   
2. Select Transparent Profile component in project graph, to open component configuration and configure as illustrated in the following figure.

   

Generating a Code

For more details on code generation, refer to MPLAB Code Configurator(MCC) Code Generation

Files and Routines Automatically Generated by the MCC

After generating the program source from MCC interface by clicking Generate Code, the BLE configuration can be found in the following project directories

The OSAL, RF System, BLE System initialization routine executed during program initialization can be found in the project files. This initialization routine is automatically generated by the MCC

Header Files

• ble_gap.h: The header file contains BLE GAP functions and is automatically included in the app.c file
• ble_trspc.h: The header file associated with APIs and structures related to BLE Transparent Client functions for application user.

Function Calls

MCC generates and adds the code to initialize the BLE Stack GAP, GATT, L2CAP and SMP in APP_BleStackInit() function

• APP_BleStackInit() is the API that will be called inside the Applications Initial State -- APP_STATE_INIT in app.c

User Application Development

Include

• definitions.h in all the files where UART will be used to print debug information.

  Note: definitions.h is not specific to just UART peripheral, instead it must be included in all application source files where peripheral functionality will be exercised.

• User action is required as mentioned User Action

Start Scanning

// Scanning Enabled
BLE_GAP_SetScanningEnable(true, BLE_GAP_SCAN_FD_ENABLE, BLE_GAP_SCAN_MODE_OBSERVER, 1000);
// Output the status string to UART
SERCOM0_USART_Write((uint8_t *)"Scanning \r\n", 11);

This API is called in the Applications initialstate - APP_STATE_INIT in app.c. Scan duration is 100 secs

Scan Results and initiating a BLE Connection

• BLE_GAP_EVT_ADV_REPORT event is generated upon finding Advertisements on legacy channels

• BLE connection can be initiated by using the API BLE_GAP_CreateConnection(&createConnParam_t);

  // code snippet to filter scan results and initiate connection
  // Filter Devices based of Address, for this example address checking only 2 bytes
  if ((p_event->eventField.evtAdvReport.addr.addr[0] == 0xA1 && p_event->eventField.evtAdvReport.addr.addr[1] == 0xA2) ||
      (p_event->eventField.evtAdvReport.addr.addr[0] == 0xB1 && p_event->eventField.evtAdvReport.addr.addr[1] == 0xB2) ||
      (p_event->eventField.evtAdvReport.addr.addr[0] == 0xC1 && p_event->eventField.evtAdvReport.addr.addr[1] == 0xC2))
  {
      SERCOM0_USART_Write((uint8_t *)"Found Peer Node\r\n", 17);
      BLE_GAP_CreateConnParams_T createConnParam_t;
      createConnParam_t.scanInterval = 0x3C; // 37.5 ms
      createConnParam_t.scanWindow = 0x1E; // 18.75 ms
      createConnParam_t.filterPolicy = BLE_GAP_SCAN_FP_ACCEPT_ALL;
      createConnParam_t.peerAddr.addrType = p_event->eventField.evtAdvReport.addr.addrType;
      memcpy(createConnParam_t.peerAddr.addr, p_event->eventField.evtAdvReport.addr.addr, GAP_MAX_BD_ADDRESS_LEN);
      createConnParam_t.connParams.intervalMin = 0x10;
      createConnParam_t.connParams.intervalMax = 0x10;
      createConnParam_t.connParams.latency = 0;
      createConnParam_t.connParams.supervisionTimeout = 0x48;
      SERCOM0_USART_Write((uint8_t *)"Initiating Connection\r\n", 23);
      BLE_GAP_CreateConnection(&createConnParam_t);
  }

Connected and Disconnected Events

• In app_ble_handler.c BLE_GAP_EVT_CONNECTED event will be generated when a BLE connection is completed.

Connection Handler

• Connection handle associated with the peer peripheral device needs to be saved for data exchange after a BLE connection.

• p_event->eventField.evtConnect.connHandle has this information

• In Multilink application, unique connection handler's will be generated for all the peripheral links

  

  // Global Variables
  uint8_t scanAddr[12]; // var to store ASCII address
  extern uint16_t conn_hdl[3];
  extern uint8_t no_of_links;
  
  // Connected Event
  SERCOM0_USART_Write((uint8_t *)"Connected!\r\n", 12);
  conn_hdl[no_of_links] = p_event->eventField.evtConnect.connHandle;
  no_of_links++;      
  
  // Disconnected Event
  SERCOM0_USART_Write((uint8_t *)"Disconnected\r\n", 15);
  

Scan Timeout Event

• In app_ble_handler.c, BLE_GAP_EVT_SCAN_TIMEOUT event is generated when BLE Scan duration expires.

  SERCOM0_USART_Write((uint8_t *)"Scan Completed! \r\n", 18);

  

Transmit Data

• Add "APP_MSG_UART_CB" to the generated APP_MsgId_T

  
• BLE_TRSPC_SendData(conn_hdl , 1, &data); is the API to be used for sending data towards the central device.
  Note: The precompiled application example uses a UART callback to initiate the data transmission upon receiving a character on UART

Example Implementation for Transmitting the received data over UART using the BLE_TRSPC_SendData API

uint16_t conn_hdl[3] = {0xFFFF, 0xFFFF, 0xFFFF};// connection handle info captured @BLE_GAP_EVT_CONNECTED event
uint8_t no_of_links;// No of connected peripheral devices
uint8_t i = 0;// link index
uint8_t uart_data;
void uart_cb(SERCOM_USART_EVENT event, uintptr_t context)
{
  APP_Msg_T   appMsg;     
  // If RX data from UART reached threshold (previously set to 1)
  if( event == SERCOM_USART_EVENT_READ_THRESHOLD_REACHED )
  {
    // Read 1 byte data from UART
    SERCOM0_USART_Read(&uart_data, 1);
    appMsg.msgId = APP_MSG_UART_CB;
    OSAL_QUEUE_Send(&appData.appQueue, &appMsg, 0);       
  }
}
void APP_UartCBHandler()
{
    // Send the data from UART to connected device through Transparent service
    BLE_TRSPC_SendData(conn_hdl[i], 1, &uart_data);
    i++;
    if(i==no_of_links) i = 0; //reset link index    
}

////////////////////////////////////////////////////////////////////////////////////
// Register call back when data is available on UART for Peripheral Device to send
// Enable UART Read
SERCOM0_USART_ReadNotificationEnable(true, true);
// Set UART RX notification threshold to be 1
SERCOM0_USART_ReadThresholdSet(1);
// Register the UART RX callback function
SERCOM0_USART_ReadCallbackRegister(uart_cb, (uintptr_t)NULL);
////////////////////////////////////////////////////////////////////////////////////

//If msg received in Queue, handle the Queue message based on msgID
if (OSAL_QUEUE_Receive(&appData.appQueue, &appMsg, OSAL_WAIT_FOREVER))
  {
    if(p_appMsg->msgId==APP_MSG_BLE_STACK_EVT)
    {
     // Pass BLE Stack Event Message to User Application for handling
     APP_BleStackEvtHandler((STACK_Event_T *)p_appMsg->msgData);
     }
     else if(p_appMsg->msgId==APP_MSG_BLE_STACK_LOG)
     {
     // Pass BLE LOG Event Message to User Application for handling
     APP_BleStackLogHandler((BT_SYS_LogEvent_T *)p_appMsg->msgData);
      }
      else if(p_appMsg->msgId==APP_MSG_UART_CB)
      {
       // Transparent UART Client Data transfer Event
       APP_UartCBHandler();
       }                
  }

Receive Data

• BLE_TRSPC_EVT_RECEIVE_DATA is the event generated when data is sent from central device

• Users need to use the BLE_TRSPC_GetDataLength(&data_len) API to extract the length of application data received

• BLE_TRSPC_GetData(&conn_hdl, data); API is used to retrieve the data, conn_hdl is the value obtained from Connection Handler section

  Note: BLE_TRSPC_Event_T p_event structure stores the information about BLE transparent UART callback functions

Example Implementation for printing the received data from peripheral device over UART

  /* TODO: implement your application code.*/
    uint16_t data_len;
    uint8_t *data;
    // Retrieve received data length
    BLE_TRSPC_GetDataLength(p_event->eventField.onReceiveData.connHandle, &data_len);
    // Allocate memory according to data length
    data = OSAL_Malloc(data_len);
    if(data == NULL)
    break;
    // Retrieve received data
    BLE_TRSPC_GetData(p_event->eventField.onReceiveData.connHandle, data);
    // Output received data to UART
    SERCOM0_USART_Write(data, data_len);
    // Free memory
    OSAL_Free(data);

For more details on exercising various other BLE functionalities refer to BLE Stack API section.